1. Field of the Invention
The present invention relates to a copy magnification modifying apparatus for use in a copier with variable magnification, and more particularly to a copy magnification modifying apparatus capable of shifting the position of optical members such as imaging lens at the modification of the copy magnification in such a manner that the photosensitive member and the original to be copied are maintained in a mutually conjugate relationship at the selected magnification.
2. Description of Prior Arts
As already well known an image of the original with a magnification b/a (=m) can be formed on the photosensitive member under the following conditions: EQU 1/a+1/b=1/f
wherein a is the optical distance from the original to the lens, b is the optical distance from the lens to the photosensitive member and f is the focal length of the lens. In case of modifying the magnification, the above-mentioned values a and b should be modified in such a manner that the ratio b/a becomes equal to the newly selected copy magnification and that thus modified values a and b still satisfy the foregoing equation. For example in a copier in which first and second reflecting means are displaced at a speed ratio of 1:1/2 for scanning the original and performing a slit exposure of the image of the thus scanned original onto a moving photosensitive member through a lens and a slit, the above-mentioned modification of values a and b is generally achieved by a change in the mutual positional relationship between said first and second reflecting means combined with a displacement of said lens.
In such copier the positional accuracy of the movable optical members such as a lens or mirror at each selected magnification is extremely important as an eventual aberration in the optical distance a or b will result in an incorrect magnification or in an unfocused low-quality image. Consequently it is required to constantly stop the optical members such as the lens or mirror exactly at the predetermined positions and to securely maintain such members at said positions.
For example there is already known a lens displacing mechanism, as disclosed in the U.S. Pat. No. 3,884,574, in which a lens carriage mounted on a wire supported between two pulleys and selectively driven forward or backward by a reversible motor actuates, at a position corresponding to a selected magnification, one of plural microswitches provided along the displacing path of said lens carriage whereby the motor is stopped by the signal from the thus actuated microswitch to position the lens at said position. However, an exact positioning of the lens is difficult to achieve with such known mechanism, since the position detecting elements such as microswitches generally involve certain deviation in the functioning point among themselves and thus require fine adjustment for achieving an improved detection accuracy, and also since a certain fluctuation in the functioning point is unavoidable even in a single microswitch. In addition, after the power supply to the motor is interrupted by the signal from the micro-switch, the motor continues to rotate for a while by the inertia, thus resulting in an uncontrollably unstable aberration between the detecting position and the actual stopping position.
The known mechanisms disclosed in the U.S. Pat. Nos. 3,873,189 and 3,598,489 are also associated with similar drawbacks.
Also there is known a mechanism for lens positioning, as disclosed in the U.S. Pat. No. 3,614,222, in which a link mechanism resiliently connected by a spring is displaced by a rotary solenoid through a cam, whereby a lens carriage is resiliently pressed against a stopper. In such known mechanism the cam is also resiliently pressed by the spring which presses the lens carriage against the stopper. Consequently if the cam is not stopped in the exact position or is eventually displaced therefrom by the vibration, said resilient force may cause rotation of said cam, thus resulting in an unstable positioning of the lens. Particularly in a copier in which the original scanning is achieved by the reciprocating motion of the reflecting means or the original carriage, the forward and backward motions thereof tend to generate vibrations at the end points thereof which undesirably affect the cam as explained in the foregoing.